New Pb-Free Solder Alloy for

Demanding Applications

Presented by Karl Seelig, VP Technology, AIM

Why REL?

The evolution and expansion of electronics into

more harsh operating environments performing

more critical functions

Drawbacks of SAC305:

Poor drop/shock performance

High melting temperature (217-220°C)

Increased concern of tin whisker growth

Weakened strength due to thermal cycling/aging

Cosmetic appearance

Optical micrograph of free-standing Sn3.8-Ag0.7-Cu solder alloy showing

a large Ag3Sn primary precipitate.

F. Mutuku et al, Journal of ELECTRONIC

MATERIALS, Vol. 46, No. 4, 2017

Large IMC Plates in SAC Alloys

with >3% Ag

Large IMC Plates in SAC Alloys

with >3% Ag

K. Zeng et al, 2012

Fracture boundaries created by platelets in high silver alloys

reduce drop shock performance.

Tin Whiskers

SAC305

Test Method

Humidity 85%, T=60°C, t=3100hr

The coated wire was stressed by making

a “U-shaped” bend to 90 degrees.

REL22

Microstructure of REL61 & REL22 remains stable vs. SAC305

Microstructure Comparison After High

Temperature Aging 24h @150°C

SAC305 As Cast

SAC305 Aged REL22 Aged

REL22 As Cast

Bismuth (Bi) offers some benefits improvements

but has limitations.

Antimony (Sb) can increase durability.

There is a long list of grain refiners. A variety of

micro-alloy elements can be incorporated to

stabilize alloys.

How to Improve SAC305

Melting Behavior

Differential scanning calorimetry (DSC):

heating rate of 10°C/min

Thermal Behavior

Parallel Solidus and liquidus lines deviate at

3% Bi

Mechanical Behavior

Samples annealed 96hr at 125° C

SAC305 is shown for comparison

AIM’s REL22TM Alloy

REL22 is a new, patent pending, high-reliability

alloy for use in extremely harsh environments.

High reliability / high strength

Mitigates tin whisker formation

Exceptional thermal cycling performance

Melting point 210°C

Drop-in to SAC305 assembly processes

Sn/Ag/Cu/Bi/Sb/Ni/X

REL22 / 480hr@150C

REL22 Kinetics of Intermetallic Growth (IMC) 740hrs at 150°C

SAC305 / 480hr@150C

Cu3Sn

Cu3Sn introduces

brittleness.

Cu3Sn

Tensile Creep (Aged 24hrs @ 150 ⁰C)

In aged condition, RT creep performance of REL22 is superior to

SAC305. In fact, SAC305 shows very poor creep performance after

aging.

Compression Creep

High temperature creep resistance of REL22 is superior to SAC305 and REL61.

(@ 150°C 10MPa and 175°C 20MPa)

Tension Test

Tension test performed per (ASTM E8/E8M-11)

Samples aged 24hrs at 150°C

Cross head speed: 2 mm/min

REL22 shows much higher strength as compared with SAC305. Elongation of

REL22 is higher than REL61 and slightly lower than SAC305.

Thermal Cycle Testing Scope

DfR Solutions performed thermal cycling experiments

on test coupons assembled with three different solder

alloys

SAC305 and two alternative Pb-free solders (REL61 &

REL22)

Sufficient time to failure data was obtained from three

different package styles

LED, MLF and Resistor

Other packages tested had insufficient failure data or

failure data that was suspect

Both alternative Pb-free solders demonstrated

improvement in thermal cycle lifetime over SAC305

Data provided by:

Thermal Cycle Testing Samples

Eight boards were assembled for each solder alloy.

Data derived from highlighted components.

Data provided by:

Thermal Cycle Testing Setup

Thermal cycle profile (per IPC-SM-785)

-40°C to 125°C / 15 minute dwells / 20°C per min ramp

Test suspended after 3950 thermal cycles.

In-situ resistance monitoring tracked changes of

resistance with time for each component through

out the duration of the test.

Failure was taken at a number of thermal cycles

corresponding to resistance change per IPC-

9701A standard.

Data provided by:

Thermal Cycle Testing Setup

Test boards were hanged vertically with

sufficient spacing to allow adequate airflow. Data provided by:

Thermal Cycling Comparison of

Solders

Data provided by:

SAC305 REL61 REL 22

Package Cycles to

Failure (η) Cycles to

Failure (η)

Ratio to

SAC305

Cycles to

Failure (η) Ratio to

SAC305

LED 918 1165 1.3 2128 2.3

MLF156-12mm 900 1360 1.5 1884 2.1

MLF48-7mm 1623 2045 1.3 2900 1.8

MLF32-7mm 1405 2126 1.5 2815 2.0

2512 1975** 1925** 1.0 5194 2.6

Average Ratio to SAC305 (=1) 1.4 2.2

**Characteristic lifetime based on first population set (indications of mixed mode

behavior)

Thermal Cycle Test Results

Data provided by: REL22 is twice as durable as SAC305.

Thermal Cycle Test Results

Data provided by: REL61 is equal to or better than SAC305 at a lower cost.

Thermal Cycle Test Results –

LED

Data provided by:

Thermal Cycle Test Results – MLF48-

7MM

Data provided by:

Thermal Cycle Test Results – 2512

Resistor

Data provided by:

Thermal Cycle Testing REL61 and REL22 Relative to SAC305

REL61 provides ~40% improvement in thermal

cycle fatigue lifetime

Measured range is 0 to 50%

REL22 provides ~120% improvement in thermal

cycle fatigue lifetime

Measured range is 80 to 160%

Data provided by:

REL22

Wetting Balance Behavior Wetting Force

REL22 = faster – stronger wetting than SAC305 or

Sn/Ag/Bi/Sb/Ni/Cu.

Wetting Time

REL61 Wetting Comparison

REL61

SAC305

Sn/Cu/Ni

Sn/Cu/Ni

Sessile Drop Test measures surface energy via contact angle.

Lower contact angles = better wetting.

REL22 No Clean Solder Paste Test Data

REL22 vs. SAC305 No Clean Paste

BGA and QFN Voiding

NC REL22 NC SAC305

QFN Voiding Lower with REL22 Improved BGA Voiding with REL22

NC SAC305 NC REL22

REL22 vs. SAC305 vs.

Sn/Ag/Bi/Sb/Ni/Cu No Clean Paste

0603 Voiding

REL22 is more consistent than

SAC305 and Sn/Ag/Bi/Sb/Ni/Cu alloys.

REL22

Drop-in for SAC305

NC SAC305 T4 NC REL22

RT

S-1

RS

S-H

S-L

RS

S-L

-HS

-

LTA

L

Wetting QFP144 / Paste vs. Profile

REL22 No Clean Solder Paste Field Data

M8 REL22 Solder Paste vs.

Competitor SAC305 Immersion Silver Surface Finish - 1.6mm thick FR4 6 layer

M8 REL22 Solder Paste

M8 REL22 Competitor SAC305

M8 REL22 results in better wetting and smoother

solder joint appearance.

M8 REL22 Solder Paste

M8 REL22 Competitor SAC305

M8 REL22 results in better wetting and smoother solder

joint appearance.

M8 REL22 Solder Paste

M8 REL22 results in better wetting and

smoother solder joint appearance.

M8 REL22 Competitor SAC305

REL22 Alloy Summary

REL22

Summary

Enhanced durability for use in extremely harsh

environments

Mitigates tin whisker formation

High reliability / high strength

Exceptional thermal cycling performance

Reduces voiding

Melting point 210°C

Drop-in to SAC305 assembly processes

REL61

Summary

Superior barrel fill

Lower solder pot maintenance

Reduces voiding

Lower cost than SAC305 alloy

Enhanced reliability versus SAC alloys

Higher strength and hardness compared to SAC alloys

Improved thermal cycling performance

Mitigates tin whisker formation

Melting point 208-216°C

Sn/Cu/Ag/Bi

Thank You